Water Softening And Fluoride Removal From Water With The Method Of Carrier Induction Crystalization

The features of carrier induction crystallization are that a large amount of the grains of suitable carrier are added in precipitation reaction system, the grains speed reaction up, precipitates sedimentate with the grains and the time of precipitation process is shorten.Its researches have been absent in China, mainly focused on application fields and its theory researches have been absent overseas. This paper has studied the reaction kinetics and thermodynamics of silica induction CaCO3 precipitation reaction and its induction mechanism, water softening with silica induction crystallization, fluoride removal with silica induction CaF2 crystallization and its induction mechanism.The effects of Na2C03 quantity, the quantity of silica grains, stirred speed, temperature on silica induction CaCO3 precipitation reaction have been studied in stirred reactor. The concentrations of Ca2+ and pH were measured during reaction and the corresponding CO32- concentrations were calculated. The kinetics equation of silica induction CaCO3 precipitation reaction could be described as r=-k' S[Ca2+][CO32-]+b; its activation energies were 14.07KJ/mol and 31.74KJ/mol respectively when the concentration of silica grains was about 250g/L and there were no silica grains. The results demonstrate that silica induces CaCO3 precipitation reaction in the reaction kinetics.The induction times, start-up supersaturations and residual supersaturations of CaCO3 precipitation reaction have been studied in stirred reactor when there were and there were no silica grains. The induction times were determined by the transformation of the concentrations of Ca2+after precipitation agent Na2CO3 were added in reaction system. The precipitation agent quantities when induction timeswere zero, which were the precipitation agent quantities when precipitation reactions started wereobtained by the linear fitting equations of induction times and precipitation agent quantities. When the concentrations of Ca2+ were stable precipitation reactions had ended. The relevant parameters of the two situations above were measured and the start-up supersaturations and residual supersaturations were calculated. At 25℃, comparing with no silica grains, silica that the concentration of its grains was about 100g/L shortened induction times 5-6min; when the concentration of silica grains was about100g/L the start-up supersaturations was decreased from 1.6 to 1.44, when the concentrations of silica grains were about 74g/L and 250g/L the residual supersaturations were decreased from 0.421 to 0.398 and 0.371 respectively. The results demonstrate that silica induces CaCO3 precipitation reaction in the reaction thermodynamics; higher the concentration of silica grains, more stable the system when reaction has ended.The peculiarities of the surface charge of silica grains, the characteristic function group on the surface of silica grains, the absorption of Ca2+ F- on the surfaces of silica grains have been studied by electrophoresis, IR and chemical analysis. The characteristic function group on the surfaces of silica grains was hydroxyl; silica grains absorbed Ca2+ F~, and increasing pH, the quantities of Ca2+ which were absorbed on silica grains increased; in silica induction CaCO3 precipitation reaction, the silica grains had negative surface charges; in silica induction CaF2 precipitation reaction, the silica grains had positive surface charges. According to the surface complexation models, the alkalinities of precipitation systems and the tests results, the main surface charge forms of silica grains were =Xo- and = XOCa+ respectively in silica induction CaCO3and CaF2 precipitation reaction, and the forms of surface electric double layer of silica grains were (2m (=XO-) (m-x) Ca2+} 2x- xCa2+ and {m ( = XOCa+) (m-x) F-)x+x F-respectively. According to the the forms of the surface electric double layer above, when there is one ion at least in their surface electric double layer which is one of the ions of precipitates, carrier grains induce precipitation reac...